1. Field of Invention
The present invention relates to a reduction casting method. More particularly, the invention relates to a reduction casting method in which casting can be performed in a favorable state-without impairing reducing strength.
2. Description of the Related Art
There are various types of casting methods such as a gravity casting method (GDC), a low pressure die casting method (LPDC), a die casting method (DC), a squeeze casting method (SC) a thixomolding method. All of these methods perform casting by pouring molten metal into a cavity of a molding die, thereby molding the thus-poured molten metal into a predetermined shape. Among these casting methods, in a method in which an oxide film is likely to be formed on a surface of the molten metal, for example, at aluminum casting or the like, a surface tension of the molten metal is increased by the oxide film formed on the surface of the molten metal to deteriorate a flowing property, a running property and an adhesive property of the molten metal thereby causing problems of casting imperfections such as insufficient filling, a surface fold and the like.
As a method to solve these problems, the present applicant has proposed a reduction casting method which is capable of performing casting by reducing an oxide film formed on a surface of the molten metal (for example, JP-A-2001-321918). In this reduction casting method, a magnesium-nitrogen compound (Mg3N2) having a strong reducing property is prepared by using a nitrogen gas and a magnesium gas and, then, the thus-prepared magnesium-nitrogen compound is allowed to act on the molten metal of aluminum, thereby performing casting. The magnesium gas is generated in a furnace and, when the magnesium gas is introduced into a cavity, an inert gas (argon gas) is used as a carrier gas. The nitrogen gas is directly introduced into the cavity in a separate manner.
According to the above-described reduction casting method, by pouring the molten metal into the cavity of a molding die in a state in which the magnesium-nitrogen compound is deposited on a surface of the cavity of the molding die, when the molten metal comes into contact with the surface of the cavity, the oxide film formed on the surface of the molten metal is reduced by a reducing action of the magnesium-nitrogen compound to change the surface of the molten metal into pure aluminum, thereby decreasing a surface tension of the molten metal and, accordingly, enhancing a flowing property of the molten metal. As a result, a running property of the molten metal becomes advantageous whereupon a cast product which does not have a cast imperfection but has an excellent appearance deprived of a surface fold or the like can be obtained.
However, there are problems as described below in the above-described reduction casting method.
Namely, in the reduction casting method, although it is necessary to control quantities of the magnesium gas and the nitrogen gas, the magnesium gas which is obtained by heat-subliming magnesium in the furnace is in a state of high temperature (about 800° C.).
It is difficult to measure the quantity of this magnesium gas in a state of high temperature and, therefore, it is unable to precisely control quantities of both gases, and thus, problems are generated such that the quantity of the magnesium gas becomes insufficient, reduction strength is deteriorated, qualities of cast products are varied thereamong and the like.